The present invention relates to analysis of sensors and similar structures, and more particularly to process parameter sensors such as mass flowmeters, and related methods and computer program products.
Many sensor applications involve the detection of mechanical vibration or other motion. Examples of sensors that utilize such motion detection include Coriolis mass flowmeters and vibrating tube densitometers. These devices typically include a conduit or other vessel that is periodically driven, i.e., vibrated. Properties such as mass flow, density and the like associated with a material contained in the conduit or vessel may be determined by processing signals from motion transducers positioned on the containment structure, as the vibrational modes of the vibrating immaterial-filled system generally are affected by the combined mass and stiffness characteristics of the containing conduit or vessel structure and the material contained therein.
A typical Coriolis mass flowmeters includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibrational modes including, for example, simple bending, torsional, radial and coupled modes. In a typical Coriolis mass flow measurement application, each conduit is excited at resonance in one of its natural vibrational modes as a material flows through the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Exemplary Coriolis mass flowmeters are described in U.S. Pat. Nos. 4,109,524 to Smith, 4,491,025 to Smith et al., and Re. 31,450 to Smith.
A commonly used type of Coriolis mass flowmeter includes parallel U-shaped conduits that form parallel material paths. The conduits are driven by a voice coil actuator connected between the conduits near their apices. A periodic drive signal applied to the actuator causes the conduits to be excited in opposing periodic patterns.
When there is substantially zero flow through a conduit, points along the conduit tend to oscillate with approximately the same phase. When material is flowing through the conduit, however, Coriolis forces arising from the material flow tend to induce phase shifts between spatially diverse points along the length of the conduit, with the phase of the inlet end of the conduit generally lagging the driver and the phase of the outlet end of the conduit generally leading the driver. The phase shift induced between two locations on the conduit is approximately proportional to the mass flow rate of the material flowing through the conduit. This phase shift typically is measured by measuring a phase shift between motion signals produced by first and second motion transducers placed near the inlet and outlet ends of the conduit, respectively, at the excitation frequency of the mass flowmeter.
Unfortunately, the accuracy of such a phase shift measurement may be compromised by nonlinearities and asymmetries in the conduit structure, as well as by unwanted contributions to the phase shift caused by extraneous forces such as forces generated by pumps and compressors that are attached to the flowmeter, as well as pressure forces exerted by the material flowing through the flowmeter. The effects of these forces are commonly compensated for by using flowmeter designs that are balanced to reduce effects attributable to external vibration, and by using frequency domain filters, e.g., bandpass filters designed to filter out components of the motion signals away from the excitation frequency. However, mechanical filtering approaches are often limited by mechanical considerations, e.g., material limitations, mounting constraints, weight limitations, size limitations and the like, and frequency domain filtering may be ineffective at removing unwanted vibrational contributions near the excitation frequency.
In light of the foregoing, it is an object of the present invention to provide process parameter sensors and associated methods and computer program products that can more accurately measure process parameters associated with material contained in a vibrating conduit or vessel.
It is another object of the present invention to provide apparatus, methods and computer program products that can provide more accurate characterization of structural motion.
These and other objects, features and advantages are provided according to the present invention by apparatus, methods and computer program products that utilize a force filter configured to receive motion signals representing motion of a conduit, vessel or other mechanical structure, and operative to produce a force-filtered motion signal that discriminates motion attributable to a force of interest among a plurality of forces acting on the structure. In process parameter sensing embodiments, the force-filtered motion signal may be used to estimate a process parameter, such as mass flow or density, associated with a material contained in a conduit or other vessel. In other embodiments, additional mode pass and/or band pass filtering is applied to produce a spatially and/or temporally filtered motion signal that may also be used, for example, for process parameter estimation.
The present invention arises from the realization that a force filter operative to filter motion attributable to selected forces acting on a structure may be generated from a modal analysis of the structure, thus carrying the applicability of modal analysis beyond the mere identification of modal responses. In addition, such a force filter may be combined with modal and temporal filtering techniques to provide improved accuracy in motion detection.
According to an embodiment of the present invention, a process parameter associated with a material contained in a vibrating structure is estimated. A plurality of motion signals representing motion at a plurality of locations of the vibrating structure is received. The received plurality of motion signals are force filtered with a force filter to produce a force-filtered motion signal that discriminates motion attributable to a force of interest among a plurality of forces acting on the vibrating structure. A process parameter associated with the material in the vibrating structure is estimated from the force filtered motion signal. Preferably, a plurality of motion signal values is generated from the received plurality of motion signals, and force filtering comprises the step of applying a force filter matrix to the plurality of motion signal values to produce a force filtered motion signal value. A process parameter, such as mass flow, density or the like, is then estimated from the force filtered motion signal value. According to an aspect of the present invention, the force filter matrix represents a product of a frequency response function matrix for the vibrating structure, a force selectivity matrix and an inverse of the frequency response function matrix.
According to one embodiment of the present invention, the force filter may represent a function of frequency evaluated at a frequency of interest, e.g., a drive mode resonant frequency. A process parameter is estimated from the force filtered motion signal at the frequency of interest. The received motion signal may represent motion in response to an excitation of the structure at the frequency of interest.
According to other embodiments, force filtering may be combined with temporal (frequency) and modal filtering. For example, a band pass filter, e.g., a filter having a passband around a frequency of interest such as a drive mode resonant frequency, may be applied to the force filtered motion signal to produce a temporally filtered motion signal. A process parameter may then be estimated from the temporally filtered motion signal at the frequency of interest. In another embodiment, a combination of the force filter and a modal resolver, e.g., a xe2x80x9cmode passxe2x80x9d filter, is applied to the plurality of motion signals to produce a spatially and temporally filtered motion signal that discriminates motion of the structure associated with a vibrational mode of interest, and a process parameter is estimated from the spatially and temporally filtered motion signal. Force, temporal and modal filtering may also be applied in combination.
According to yet another aspect of the present invention, motion attributable to a force of interest among a plurality of forces applied to a structure is estimated. A motion signal representing motion of the structure is force filtered to produce a force filtered motion signal that discriminates motion attributable to the force of interest. Preferably, the force filtering comprises applying a force filter matrix to a motion signal value generated from the motion signal to produce a force filtered motion signal value. The force filter matrix may represent a product of a frequency response function matrix for the structure, a force selectivity matrix and an inverse of the frequency response function matrix.
According to another aspect of the present invention, a process parameter sensor comprises a structure configured to contain a material, and a plurality of motion transducers operatively associated with the structure and operative to produce a plurality of motion signals representing motion of the structure. A force filter is responsive to the plurality of motion signals and operative to produce a force-filtered motion signal therefrom that discriminates motion attributable to a force of interest among a plurality of forces acting on the structure. A process parameter estimator is operative to estimate a process parameter associated with material in the structure responsive to the force filtered motion signal. Preferably, the sensor includes means for generating a plurality of motion signal values from the plurality of motion signals, the force filter comprises means for applying a force filter matrix to the plurality of motion signal values to produce a force filtered motion signal value, and the process parameter estimator comprises means for estimating a process parameter from the force filtered motion signal value. The force filter matrix may represent a product of a frequency response function matrix for the structure, a force selectivity-matrix and an inverse of the frequency response function matrix.
According to yet another aspect of the present invention, a computer program product for determining motion attributable to a force of interest among a plurality of forces applied to a structure is provided. The computer program product comprises a computer-readable storage medium having computer-readable program code means embodied in the medium, the computer-readable program code means including computer-readable program code means for force filtering a motion signal representing motion of the structure to produce a force filtered motion signal that discriminates motion attributable to the force of interest. The computer-readable program code means preferably comprises computer-readable program code means for applying a force filter matrix to a motion signal value to produce a force filtered motion signal value, wherein the force filter matrix may represent a product of a frequency response function matrix for the structure, a force selectivity matrix and an inverse of the frequency response function matrix.
Improved methods, apparatus, and computer program products for estimating motion in a structure, such as a Coriolis mass flowmeter conduit, are thereby provided.